Depolarization potentials created during a muscle fiber contraction generate an electrical field gradient which propagates in a direction along the fibers throughout the volume conductor comprised of the muscle, the surrounding tissue, and skin layers. Electrodes placed on the skins surface allow for the non-invasive detection of this electrical field gradient providing the temporal summation of the propagating depolarization potentials of the active muscle fibers in the underlying vicinity of the electrode. The resulting voltage on the skin's surface is termed the surface electromyographic signal (sEMG).
In order to measure this voltage, an electrolytic interface may be formed between the electrolytes in the subcutaneous tissue and the ohmic conductive surface of the electrode contact attached to the skins surface. The primary electrical conduit between the subcutaneous volume conductor and the skins surface is established via the sweat ducts which pass through the non-conductive stratum corneum so that sweat and moisture from the underlying sweat glands are deposited onto the skins surface completing the electrolytic interface.
The electrolytic interface is provided by disassociated ions from the electrolyte forming a layer on the conductive electrode contact surface (Nernst polarization or contact half-cell potential). Depending on the chemical composition, concentration of the electrolytes on the skin, and the composition of the electrode contact metal, the half-cell potentials can range in amplitude to several hundred millivolts. Signal potentials emanating from the muscle in the underlying tissue are conveyed via ionic transport through the electrolyte to the exposed conductive contact surface of the electrode. The signal amplitude is typically several orders of magnitude smaller than the half-cell potential and ranges from 10 microvolts to 5 millivolts. The resultant voltage sensed by the electrode contact is therefore the electrical summation of the signal potential and contact half-cell potential.
When the electrolytic skin interface of the electrode is mechanically disturbed due to relative movement or pressure changes between the tissue and conductive surface of the electrode, the effective concentration of the electrolytes can be altered so that the resultant half-cell potential amplitude is modulated by the mechanical disturbance. The modulation of hall-cell potential is termed “movement artifact” and typically arises from rapid body movements, or objects or clothing coming into contact with the sensor case housing the electrodes.
Movement artifact can be particularly problematic as the change in half-cell potential can exhibit large (>50 mV) voltage deviations which overwhelm the amplitude of the sEMG signal. An additional source of movement artifact is due to the triboelectric charge that can accumulate on the non-conducting stratum corneum as a result of walking on carpet or contact with certain fabrics under low humidity conditions. This effect can be especially problematic when the electrolytic skin interface exhibits high impedance resulting from the lack of suitable moisture between the electrode contact and the skin. This impedance can reach tens of megohms for contacts with an area of 1 mm squared placed on unprepared skin.
Prior art techniques developed to address reduction in movement artifact focus on the materials used to fabricate the conductive surface of the electrode contact. Highly conductive aqueous salt solutions or hydrophilic gels applied to the contacts surface act to improve the electrolytic skin interface by augmenting the nature moisture present on the skins surface to stabilize and reduce skin impedance, however these materials can create large half-cell potentials, and if not properly applied, can leak to form low impedance bridges between electrode contacts thereby “shorting out” the desired bio-potential signal. Electrodes formed as an insulating capacitive plate overcome some of the problems associated with half-cell potential created when using ohmic electrodes, but are subject to the triboelectric charge effect on dry stratum corneum, and therefore subject to movement artifact and static discharge when placed under clothing. The class of “dry” electrodes formed from silver metal or silver coated plastic contacts falls in between the two extremes of gel and capacitive electrode designs. They have higher initial impedance than gel, relying on the inherent moisture present on the skin to form the electrolytic interface. They are typically formed of pure silver or silver-silver chloride.
Regardless of the contact materials used, it is known that the configuration of a sensor designed to detect sEMG signals may include two disposable electrode contact discs, one for each signal input placed singularly, or in pairs, mounted on a flexible non-conductive adhesive pad so that the applied contact conforms to the underlying body surface. The electrodes may be attached by snaps or spring loaded clips and connected to remote electronic circuitry via a shielded insulated tethered cable.
Characteristically, the sensor includes the two signal contacts located over the muscle and a third “reference” contact located at an electrically inactive location on the body. In some sensors the two signal and reference contacts are placed on the same insulating pad in the form of an equilateral triangle. The orientation for the signal input contact pair may be in a direction parallel to the muscle fiber. The recording configuration may be the differential configuration where the voltage at each signal input contact is measured with respect the third reference contact and subtracted using a differential pre-amplifier circuit. In this way, any voltages common to both electrodes such as half-cell potentials and line interference effectively subtract to zero for an ideal amplifier. However, in compliant or flexible electrode skin interface designs, disturbances to the electrode interface induced from contact forces applied directly to the interface or induced from shear forces applied to the skin are likely to cause an unequal localized disruption of the electrolyte junction half-cell potential of each electrode contact. This unequal change in half-cell potentials can not be removed by differential subtraction and as a result generates a movement artifact signal.
As further background to the system described herein, sEMG sensors with or without onboard signal conditioning circuitry are typically tethered via a cable which acts to power and convey the output signal from the sensor to external instrumentation. With the advent of wireless technology, wireless versions of sEMG sensors are becoming an alternative to tethered sensors. These sensors typically dispense with the reference ground contact due to the fact that unlike tethered designs, they electrically “float” with respect to earth ground. These are termed “reference free” designs. The two electrode sensor can be configured either as a mono-polar, or as a differential amplifier with an internally generated electrical reference. Both configurations measure the voltage difference measured between the two contacts. A typical wireless sensor consists of two gel filled disk contacts mounted on disposable adhesive backed pad. Snap leads connect the electrodes via a short cable to the inputs of a wireless module containing the sEMG signal conditioning electronics.
All of the aforementioned electrode contact and sensor configurations described as prior art, whether tethered or wireless, are subject to the effects of movement induced artifact, due to inability to electrically stabilize the electrolytic interface during a mechanical disturbance to the electrode contacts. The commonly-applied technique to mitigate sensitivity to movement artifact is to decrease and stabilize the impedance of the skin-electrode interface using conductive aqueous salt solutions or hydrophilic gels applied to disposable electrodes. When combined with a differential recording configuration using a remote reference, these solutions are partially effective but incur the problems associated with application of pastes or dehydration of gels over time, limiting their useful self-life.
The problems of artifact and sensitivity to electro-static fields are especially severe for wireless “reference free” sEMG sensor designs which use only two contacts. Wireless solutions utilizing differential recording from a triangular configuration of three disposable electrodes, while less affected by electro-static fields, are still subject to movement induced artifacts.
Accordingly, it would be desirable to provide an electrode configuration which when applied to unprepared skin, can suitably detect muscle signals during highly dynamic activities, while suppressing associated movement artifact. Furthermore, the electrode configuration would be directly applicable to wireless sEMG sensor technologies which provide the inherent benefit of un-tethered, unencumbered measurement from muscles during these types of activities.